In recent years, GaN-based compound semiconductor materials have become notable as semiconductor materials for short-wavelength light emitting devices. GaN-based compound semiconductors are formed by Metal Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE), using various oxides such as sapphire single crystals or Group III-V compounds as substrates.
The properties of GaN-based compound semiconductor materials include low current diffusion in the transverse direction. This is attributed to dislocations threading from the substrate to the surface, which are abundant in epitaxial crystals, but the details thereof are not fully understood. In p-type GaN-based compound semiconductors, the resistivity is higher than the resistivity in n-type GaN-based compound semiconductors, and with mere lamination of a metal on a portion of the surface there is essentially no transverse spread of current in the p-layer, and light emission only occurs directly under the positive electrode in an LED structure having a pn junction.
Thus, it is common to employ a light-permeable positive electrode wherein light emitted directly under the positive electrode is extracted outward through the positive electrode. A common technique used commercially for a positive electrode is to laminate Ni and Au to about a few tens of nanometers each on the p-layer, and then heat the laminate in an oxygen atmosphere for alloying treatment (see Japanese Patent No. 2803742). Conductive transparent oxides such as ITO are also employed. A conductive transparent oxide can be formed into a film by sputtering or vapor deposition.
However, the problem of current diffusion still has not been completely solved using such transparent electrodes. One reason for this is insufficient current diffusion within the n-layer. In order to increase current diffusion in the n-layer it is necessary either to lower the resistivity of the n-layer or increase the film thickness to lower the resistance value. However, such strategies for lowering the resistance value also reduce the crystallinity. Reduced crystallinity tends to result in impaired characteristics such as aging-induced leaking and lower electrostatic resistance.
There have been proposed methods of reducing the distance between the n-electrode and p-electrode by surrounding the transparent electrode situated at the center with the other electrode (see Japanese Unexamined Patent Publication Nos. 9-97922 and 8-340131). Still, in such a structure with the transparent electrode at the center surrounded with the n-electrode, emitted light is blocked by the surrounding electrode and fails to be satisfactorily extracted outward.
As the electrode positioning used for chips with relatively large areas, there are disclosed techniques for forming structures in which the negative electrode and positive electrode are alternately embedded in a comb-like fashion (see Japanese Unexamined Patent Publication Nos. 5-335622 and 2003-133589). However, when such a configuration is used in a semiconductor that exhibits the aforementioned problem of current spread in the transverse direction of the p-layer and n-layer, emitted light is still blocked by the electrode and fails to be satisfactorily extracted outward.